Method for producing defect free composite membranes

ABSTRACT

A defect free semipermeable composite membrane having excellent integrity and high water permeability is provided. Said composite membrane comprises an inside support layer to provide sufficient mechanical strength; an outside barrier layer to provide selective separation; and a middle layer to provide both physical adhesion and chemical binding between said support and said barrier layer to bond them together. Three different methods for making said defect free composite membrane are discovered. These methods have been successfully utilized to produce high quality coatings and defect free composite membranes, which are independent of chemical composition and physical structure of said support. In the present invention, ultrasonic sonication is discovered to be effective to speed up the phase inversion process of a membrane casting solution, thus allows produce a composite membrane at a speed much higher than those disclosed in the prior art. Said defect free composite membranes have broad applications, ranging from filtration of fruit juice, wine and milk to biotech down stream processing and purification of drinking water, municipal and industrial wastewater.

CROSS-REFFERENCE TO RELATED APPLICATIONS

This application is Divisional of U.S. patent application Ser. No.11/214,431, filed Aug. 29, 2005, Pub. No. US2006/0000766 A1 publishedJan. 5, 2006, which is also Divisional of U.S. patent application Ser.No. 10/620,715, filed Jul. 16, 2003, and is hereby incorporated byreference in its entirety, and which is the national phase ofInternational Application No. PCT/US2004/022502, filed Jul. 14, 2004,International Publication No. WO 2005/009580 A2, published Feb. 3, 2005.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to the field of semipermeable membranes,which are useful in liquid and gas separation by filtration, dialysisand the like.

An industrial membrane should have a high permeability, sufficientmechanical strength and excellent chemical stability to give a highproductivity and a long service time. The membrane chemical stability ismainly determined by chemical composition of membrane materials.Membrane permeability and mechanical strength not only depend onmembrane chemical composition, but also strongly depend on membranephysical structure, which is primarily determined by the processutilized to make the membrane.

It is well know that the thinner the membrane, the higher the membranepermeability, however, the thinner the membrane, the weaker themembrane. In order to obtain both high membrane permeability andsufficient mechanical strength, a composite membrane approach has beenused. In general, a composite membrane comprises a thin film layer andat least one support layer. The thin film layer (referred to herein as amembrane) provides a separation barrier, which allows minimize flowresistance to increase permeability, and the support provides mechanicalstrength to a composite membrane.

U.S. Pat. No. 4,061,821 discloses a method of coating a hollow braidwith a polyacrylonitrile solution to form a braid-supported hollow fibermembrane, which shows a higher mechanical strength and a higherstability to hot water treatment than the self-supportedpolyacrylonitrile membrane having no braid support.

In water treatment, a bleach containing sodium hypochlorite as a freechlorine source is often used for membrane cleaning and waterdisinfections. Polyacrylonitrile based membranes disclosed in U.S. Pat.No. 4,061,821 are not stable to chlorine attack. However, this problemcan be overcome by using a polyvinylidene fluoride (PVDF) basedmembrane, which is relatively stable to free chlorine attack. U.S. Pat.No. 5,472,607 discloses a method of coating a tubular braid with a PVDFsolution to form a braid reinforced hollow fiber membrane. The PVDFsolution only coats the outside surface of the braid without penetratinginto the braid wall. The membranes obtained are stable to 2000 ppm offree chlorine at ambient temperature. Unfortunately, the membranesdisclosed in U.S. Pat. No. 5,472,607 have very low water permeability.U.S. Pat. No. 5,914,039 granted to the same group of inventors disclosesa method, in which partially hydrolyzed poly(vinyl acetate) and calcinedα-alumina particles are added to a braid supported PVDF membrane, whichshows a higher pure water permeability than the corresponding membranehaving no calcined α-alumina particles. However, this membrane shows asevere fouling problem in wastewater treatment, because the calcinedα-alumina particles in the membrane are excellent absorbents, which havevery large surface area and a high tendency to absorb impurities fromfeed solutions to reduce membrane flux. To minimize the membrane foulingproblem, the above membrane is operated under frequent back flush, whichis often found to cause membrane delaminated, i.e. the membrane ispeeled off the braid surface by back flush. A variety of materials, suchas polyester, fiberglass and nylon, are used to make a tubular hollowbraid. It is found that fiber glass braid shows a more severe membranedelamination problem than polyester and nylon braids due to poormembrane adhesion to the surface of fiber glass braid. U.S. Pat. No.6,354,444 discloses a physical method to tackle membrane delaminationproblem, i.e., using different type of braid as a membrane support,which has different braiding patterns, such as regular, hercules anddiamond. It is found that the diamond braid having tighter weaves thanthe regular and hercules braids gives an improved membrane adhesion.However, the membrane delamination problem remains.

In the prior art, the coating quality strongly depends on the braidquality. For example, broken fibers protruding from the surface oftubular braid caused an uneven coating around the broken fibers to formpinholes. According to U.S. Pat. No. 6,354,444, a braid used as amembrane support must have proper weaves. Too open weave causes thefilament (fiber) embedded by a polymeric coating material to give a lowmembrane permeability, and too tight weave causes poor membrane adhesionto the braid surface, the membrane is often found to be peeled off thebraid surface by back flush.

Furthermore, the membrane casting solutions in the prior art is unstableand difficult to make to give a poor reproducibility. For example, ahydrophilic component (HPVA) used in a membrane casting solution in U.S.Pat. Nos. 5,472,607, 5,914,039, and 6,354,444 is made by a partialhydrolysis of poly(vinyl acetate). Concentrated sulfuric acid is used asa catalyst, the reaction is carried out over a long time period at anelevated temperature. The degree of hydrolysis is very difficult tocontrol, and varies from batch to batch. U.S. Pat. No. 6,024,872discloses a method of making a dope containing calcined α-aluminaparticles, which causes an even more severe problem than the partialhydrolysis of poly(vinyl acetate), because the calcined α-aluminaparticles are partially precipitated out of the membrane castingsolution during storage, the degree of precipitation varies with time,resulting in a non uniform coating and poor membrane reproducibility.

The highest speed disclosed in the prior art for coating a braid is 40ft/min, it is relatively low and should be improved for a higherproductivity.

The present invention is aimed to solve the problems that were notsolved in the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a defect freesemipermeable composite membrane having its barrier layer stronglybonded to the support.

It is another object of the present invention to provide a method, whichcan strengthen the binding between the barrier layer and the support ofa composite membrane to prevent the membrane from peeling off thesupport during back flush cleaning.

It is another object of the present invention to provide a spinneret,which has a multiple inlets allowing simultaneously coating a tubularsupport with multiple layers to give a defect free composite membrane.

It is another object of the present invention to provide an effectivecoating method to give a high quality coating and a defect freemembrane, which is independent of chemical composition and physicalstructure of the support, especially, independent of braiding patterns,such as regular, hercules and diamond.

It is another object of the present invention to make a stable andreproducible membrane casting solution under a mild condition, which cangive a hydrophilic high flux membrane.

It is another object of the present invention to provide a method, whichcan enhance mass transfer and speed up phase inversion process of amembrane casting solution to produce a composite membrane at a speedhigher than those disclosed in the prior art.

A robust composite membrane has been discovered in the presentinvention, the membrane does not burst or delaminate from support undera back pressure higher than 100 psi because of strong physical adhesionand chemical binding between the membrane and the support. Two differentmethods have been discovered to provide such a strong binding: (1)adding a permeable adhesive layer between the membrane and the supportto bind them together during membrane formation; and (2) applying anadhesive from the support side of a composite membrane after it isformed to bind the membrane and the support together. In contrast to thephysical attachment disclosed in the prior art, the two methodsdiscovered in the present invention provide both chemical binding andphysical adhesion between the membrane and the support. Therefore thecomposite membranes discovered in the present invention are strongerthan those disclosed in prior art.

According to the present invention, a new type of spinneret isdisclosed, which has at least two inlets to provide different coatingsolutions for simultaneously coating a support with multiple layers toform a defect free composite hollow fiber membrane.

According to the present invention, a method is disclosed for providinga high quality coating on a variety of supports, such as braid, knittedtube, and extruded hollow fiber. In contrast to the prior art, a highquality coating discovered in the present invention is independent ofchemical composition and physical structure of support. This is achievedby simultaneously coating a support with multiple layers. The firstcoating layer not only covers all the defects and roughness of thesupport, but also provides a smooth surface and strong adhesion for asecond coating layer.

According to the present invention, a method is disclosed to make astable membrane casting solution, which comprises a hydrophobic polymeras a major component, a hydrophilic polymer as a minor component andboth inorganic and organic additives as pore formers. The hydrophobicpolymer provides the membrane with excellent chemical stability, thehydrophilic polymer provides a hydrophilic surface property and both theinorganic and organic pore formers provide high porosity. In contrast tothe prior art, there are no hydrolysis of poly(vinyl acetate) and nocalcined α-alumina particles in the present invention, thus the membranecasting solution obtained is very stable during storage. It gives a muchbetter control in coating quality and reproducibility than the priorart. The use of a commercially available hydrophilic polymer in thepresent invention allows avoid the time consuming hydrolysis reactiondisclosed in the prior art, thus to increase productivity and lowerproduction cost.

Furthermore, in the present invention the ultrasonic sonication isutilized in the coagulation bath, primary and secondary leaching bathsto enhance mass transfer and to speed up phase inversion process. Theuse of ultrasonic sonication in the present invention allows produce acomposite membrane at a speed higher than those disclosed in the priorart.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be illustrated with the assistance of thefollowing drawings.

FIG. 1 is the drawing of a new type of spinneret of the presentinvention.

FIG. 2 is the cross section view of a composite hollow fiber membrane ofthe present invention.

FIG. 3 is the schematic illustration of a novel process for making acomposite membrane in the present invention.

FIG. 4 is the filtered and concentrated orange juice.

FIG. 5 is the filtered and concentrated lemon juice.

FIG. 6 is the filtered and concentrated milk.

FIG. 7 is the filtered and concentrated soymilk.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has a number of features, which are more advancedthan the prior art. These advances are described in details in thissection and defined in the appended claims.

According to the present invention, a novel composite hollow fibermembrane is obtained by coating a tubular support with multiple layersusing a specially designed spinneret 1 as shown in FIG. 1. The spinnerethas two inlets 17 and 19, for two different coating solutions. A tubularsupport 1 enters the spinneret through a small hole 2 located at the topof the spinneret, and hole 9 in the middle. Both holes 2 and 9 playimportant roles in controlling the tension and alignment of support 1. Atubular support may deform during shipment and storage to give an ovalcross section, holes 2 and 9 can make the deformed tubular supportrestored to its original circular shape. When the support 1 passesthrough a small hole 9, it is coated with the first coating solution inchamber 11. After passing through another hole 13, the support coatedwith the first coating solution is subsequently coated with the secondsolution in chamber 16. The coating thickness is controlled by hole 15.The first coating solution may or may not be the same as the secondcoating solution depending on desired membrane performance.

A typical cross section view 2 of a composite membrane obtained in thepresent invention is schematically shown in FIG. 2. It comprises threedifferent layers. Inside layer 20, represents a porous support, whichprovides mechanical strength to a composite membrane. Outside layer 21represents a membrane, which provides a barrier for separation. Middlelayer 22 represents a permeable binding layer, which provides bindingbetween the support and the membrane. The beauty of multiple coatinglayers in the present invention is that the first coating layer not onlycovers the surface roughness and defects of the support to provide asmooth surface for second coating, but also provides binding between thesupport and the membrane to bond them together. The first coating layeris very porous, thus has negligible resistance to liquid permeation.

A system utilized for coating a tubular support to form a compositehollow fiber membrane is schematically shown in FIG. 3. The fibercoating system comprises a fiber unwound station 23, a set of rollers24-26, spinneret 1, coagulation bath 29, primary leaching bath 38,secondary leaching bath 39, a set of motorized rollers 30-35, and twofiber take-up wheels 36 and 37, immersed in the secondary leaching bath39. An ultrasonic sonicator is installed in the coagulation bath 29, itcan also be installed in the primary and secondary leaching bathsdepending on needs. A laser scan micrometer can be installed betweenroller 34 and 35 to monitor fiber diameter and consistency duringmembrane formation.

A general process for making a composite hollow fiber membrane is alsoillustrated in FIG. 3. A tubular support 1 in the present invention isselected from the group consisting of braid, knitted tube, extrudedhollow fiber and any other hollow tubular material, which has eithersmooth or rough surface. A tubular support 1 from spool 23 is guidedthrough a set of rollers 24-26, which control tension prior to coating.A tubular support 1 is coated by two polymer solutions when passingthrough spinneret 1. A detail illustration of coating process inside thespinneret is given in FIG. 1. When the support 1 passes through a smallhole 9, it is coated in chamber 11 by the first coating solution, whichis selected from the group consisting of epoxy, polyurethane, silicone,other adhesives and other polymer solutions, which have excellentcompatibility with both the braid and membrane to bond them together. InExample 1, the first coating solution is the same as the second coatingsolution. In Example 2, the first coating solution is a proprietaryadhesive specially formulated for strengthening the binding between themembrane and the support. The adhesive layer covers all the defects ofthe tubular support. The first coating provides a smooth surface andstrong binding for the second coating layer. After passing through hole13, the adhesive coated braid is subsequently coated with the secondcoating solution in chamber 16 of spinneret 1. In Examples 1-4 of thepresent invention, the second coating solution contains a fluoropolymeras a major component and a hydrophilic polymer as a minor component. Thecoating thickness is controlled by hole 15. The polymer coated braid isallowed travel a very short distance, such as 4 inches, in air beforeentering coagulation bath 29, where polymer phase inversion from liquidto solid takes place to form a composite hollow fiber membrane. Anultrasonic device 27, which can generate ultrasonic vibration, isinstalled in coagulation bath 29 to enhance mass transfer between thecoagulation media and newly formed membrane to efficiently removesolvent and additives from the membrane. The solidified membrane istransferred from coagulation bath 29 into a primary leaching bath 38 viaa roller 31 above gelation bath 29. The primary leaching bath 38 has twomotorized rollers 32 and 33. The fiber is wrapped two dozen times aroundtwo parallel rollers 32 and 33 to leach the residual solvent andadditives out of the membrane. Then, the fiber is allowed pass overroller 34 and 35 and is finally collected by a take-up wheel immersed inwater in a secondary leaching tank 39, the remaining chemical residualsare removed from the membrane at this stage. A laser scan micrometer canbe installed between roller 34 and 35 to monitor fiber size andconsistency. The signal obtained from the laser scan micrometer can besent back to the dope delivery system to control dope delivery rate.Ultrasonic device can be installed in both the primary and the secondaryleaching bath 38 and 39, respectively. The ultrasonic device installedin the coagulation bath 29 has significantly enhanced mass transfer andspeeded up the phase inversion from liquid to solid during membraneformation. Thus, a composite hollow fiber membrane in the presentinvention has been produced at a speed much faster than those disclosedin the prior art.

Example 1 illustrates a basic process for making a braid supportedhollow fiber membrane in the present invention.

A membrane casting solution (referred to herein as Dope I) is made bydissolving 13 parts by weight of PVDF, 5 parts by weight ofpolyvinylpyrrolidone (PVP), 5 parts by weight of aluminum chloridehexahydrate (AlCl₃.6H₂O), and 2 parts by weight of poly(vinylbutyral-co-vinyl alcohol-vinyl acetate) in 75 parts by weight of1-methyl-2-pyrrolidinone (NMP) as a solvent.

A composite hollow fiber membrane is prepared by coating a braid twicewith Dope I using a process shown in FIG. 3. A braid used as a membranesupport has a tubular geometry and a curved surface, its cross section20 is schematically shown in FIG. 2. The first coating layer 22 coversthe rough surface of braid and provides a smooth surface for a secondcoating as displayed in FIG. 2. The second coating layer 21 seals anydefect that the braid may still have after the first coating to form adefect free composite membrane as shown in FIG. 2.

In Example 1, ultrasonic sonication is applied to the coagulation bathto enhance mass transfer and to speed up phase inversion process. Acomposite hollow fiber membrane is produced at a speed of 60 ft/min. Acomposite membrane obtained has an outside diameter of 78 mil, a waterpermeability of 50 gfd/psi measured at 10 psi transmembrane pressure.

The membrane burst pressure is defined as the pressure at which themembrane is ruptured. This parameter is very important, because themembrane is often cleaned by back flush, the membrane may delaminate(i.e., peel off) from the support if the membrane burst pressure islower than the pressure applied for back flush cleaning. The compositehollow fiber membrane obtained from Example 1 has a burst pressure of 40psi, it is not very high, but sufficient for most of filtrationapplications.

The membrane obtained from Example 1 is useful for a variety ofapplications, such as water purification and filtration of wine, juiceand milk. The 100% orange juice containing suspended particles, which issold in a local supermarket under a brand name of Tropicana PurePremium, is filtered by the membrane obtained from Example 1 to give ayellow retentate (concentrate) and clear filtrate (permeate) as shown inFIG. 4, the filtrate is full of aroma and less sweet than the original100% juice to become a delicious diet orange juice. A similar result isobtained from filtering concentrated lemon juice containing suspendedparticles to give a clear permeate and white cloudy concentrate as shownin FIG. 5, the filtrate is a delicious diet lemon juice.

The membrane obtained from Example 1 has been utilized for concentrationof milk and soymilk. Whole milk obtained from a local supermarket isfiltered to give a clear permeate as shown in FIG. 6, the concentratedmilk obtained can be used to make cheese and other dairy products. Asimilar result is obtained when filtering soymilk sold in a localsupermarket under a brand name of Silk, the filtrate obtained is a lightyellow clear solution, the retentate obtained is a white milky solutionas shown in FIG. 7.

The surface water obtained from Canobie Lake, Salem, N.H., is filteredwith the membrane obtained from Example 1 to give potable water, whichis as clear as a purified bottled water purchased from a localsupermarket. The unfiltered Canobie Lake water is darker than both thefiltered water and the bottled water because the lake water containssuspended particles and other soluble impurities. For comparison, theinventor drinks a glass of filtered Canobie Lake water and a glass ofpure water purchased from a local supermarket, no difference in taste isdetected.

Sewage water obtained from a local sewer system, which has a black colorand stinky smell, is filtered with the membrane obtained from Example 1.The filtered sewage water is as clear as the drinking water, has no odorand is dischargeable.

The membrane obtained from Example 1 is further utilized to filter whiteand red wine. In order to mimic unfiltered wine, green grape and Italianwhite wine, BELLA SERA PINOT GRIGIO, was blended together with a kitchenblender to give a wine mixture containing suspended grape particles. Thewine mixture is filtered with membrane to give a white wine permeabilityof 26.3 gfd/psi and a filtered sparkling white wine which looksidentical to the bottled wine, the suspended grape particles arecompletely removed from the wine by membrane filtration.

Similarly, red grape and French red wine, BARTON & GUESTIER MERLOT, areblended together with a kitchen blender to give a red wine mixturecontaining grape particles. After membrane filtration, sparkling redwine is obtained and looks identical to the bottled wine, indicatingthat the membrane has a right pore size to let red pigment of the redwine freely passing through the membrane while removing the suspendedgrape particles.

A post treatment is carried out by immersing the membrane obtained fromExample 1 in an aqueous solution containing 10,000 ppm sodiumhypochlorite at ambient temperature for 48 hours. After this posttreatment, pure water permeability is increased from 50 to 141 gfd/psi,Canobie Lake water permeability increased from 32 to 38 gfd/psi, andsewage water permeability increased from 11 to 19 gfd/psi. No differencein permeate quality is detected compared to those obtained from theuntreated membrane.

Example 2 illustrates an impact of adding an adhesive layer between thesupport and the membrane on membrane performance, in particular onmembrane burst pressure, which is a critical parameter to evaluatemembrane integrity.

In Example 2, a braid is first coated with a proprietary adhesivespecially formulated for strengthening binding between the braid and themembrane, then coated with Dope I obtained from Example 1 using aspinneret shown in FIG. 1 and a process depicted in FIG. 3 to give acomposite hollow fiber membrane which has three different layers asschematically shown in FIG. 2. The inside thick layer 20 represents thebraid, the middle thin layer 22 represents the adhesive, and the outsidelayer 21 represents the membrane. The membrane obtained is treated at80° C. for 8 hours to give a burst pressure of 82 psi, which is abouttwice as high as that of the membrane (control) obtained from Example 1.Compared to the control obtained from Example 1, adding an adhesivelayer between the braid and the membrane resulted in a lower waterpermeability of 17 gfd/psi.

A post treatment with 10,000 ppm sodium hypochlorite aqueous solution atambient temperature increases the water permeability from 17 to 42gfd/psi. This chlorine treated membrane is utilized to filter surfacewater from Canobie Lake to give clean potable water. The adhesivereinforced membrane is also used to filter sewage water to give a sewagewater permeability of 8 gfd/psi, the filtered sewage water is as clearas drinking water and is dischargeable.

The beauty of adding an adhesive layer between the support and themembrane is that the adhesive layer not only covers the defects androughness at braid surface as shown in FIG. 2 to provide a smoothsurface for the second coating, but also strengthens the binding betweenthe support and the membrane.

An alternative approach to strengthen the binding between the membraneand the support is illustrated in Example 3.

The composite hollow fiber membrane obtained from Example 1 is filledwith a proprietary adhesive for a short time period to impregnate thebraid and the membrane from inside. The excess amount of adhesive isremoved from the membrane. The membrane impregnated with adhesive isheated at 80° C. for 8 hours. The composite membrane obtained fromExample 3 has an outside diameter of 78 mil. The adhesive reinforcedmembrane does not rupture when applying 100 psi pressure from the insideof membrane, indicating that the membrane has a burst pressure at least100 psi which is much higher than that of the control obtained fromExample 1. Consequently, the water permeability is reduced from 50 to 20gfd/psi compared to the control.

A post treatment with 10,000 ppm sodium hypochlorite aqueous solution atambient temperature for 48 hours increases pure water permeability from20 to 55 gfd/psi. This chlorine treated membrane is used to filterCanobie Lake water to give clear potable water with a permeability of 22gfd/psi. The chlorine treated membrane is also used to filter sewagewater to give clean dischargeable water with a permeability of 12gfd/psi.

Example 4 further illustrates the impact of multiple layer coating onmembrane performance using a dope containing poly(vinylidenefluoride-co-hexafluropropylene) (PVDF-HPF). PVDF-HFP is more stable thanPVDF.

In Example 4, a membrane casting solution (Dope II) is made bydissolving 14 parts by weight of PVDF-HPF, 5 parts by weight of PVP, 5parts by weight of aluminum chloride hexahydrate (AlCl₃.6H₂O), and 2parts by weight of poly(acrylonitrile-co-vinylidenechloride-co-methylmethacrylate) in 74 parts by weight of NMP as asolvent. A composite hollow fiber membrane is prepared using a spinneretshown in FIG. 1 and a process depicted in FIG. 3. The first coatingcovers surface roughness to provide a smooth surface for a secondcoating; while the second coating seals any defect, that the braid maystill have after the first coating, to give a defect free membrane.

A composite membrane obtained from Example 4 has an outside diameter of78 mil, a burst pressure of 36 psi, a water permeability of 26 gfd/psimeasured at 10 psi transmembrane pressure, and a rejection of 90%towards poly(ethylene oxide) molecular weight marker having an averagemolecular weight of 200 kD. The membrane shows excellent performance infiltration of orange and lemon juice and in concentration of milk andsoymilk, the details are given in Table 4 of the present invention.

A post treatment with 10,000 ppm sodium hypochlorite aqueous solution atambient temperature increases the water permeability from 26 to 51gfd/psi. This chlorine treated membrane is utilized to filter surfacewater from Canobie Lake to give clean potable water with a permeabilityof 27 gfd/psi. The chlorine treated membrane is also used to filtersewage water to give clean dischargeable water with a permeability of 10gfd/psi.

The beauty of simultaneously coating the support twice with the samedope is to completely eliminate the defect from the membrane withoutadding extra cost to manufacturing process compared to a single layercoated membrane disclosed in the prior art.

The use of a commercially available poly(vinyl butyral-co-vinylalcohol-vinyl acetate) in the present invention to providehydrophilicity to the membrane allows avoid the time consuminghydrolysis reaction of poly(vinyl acetate) in the prior art. The use ofaluminum chloride hexahydrate and polyvinylpyrrolidone in the presentinvention to provide the membrane with high porosity allows avoid thedope instability problem caused by precipitation of calcined α-aluminaparticles in the prior art. The use of ultrasonic sonication in thepresent invention to speed up phase inversion of a membrane coatingsolution from liquid to solid allows produce a composite hollow fibermembrane at a speed higher than those disclosed in the prior art. Theuse of multiple-layer coating method in the present invention allowsproduce a strong, durable and defect free composite membrane. Therefore,the present invention produces more superior composite membranes andprovides more advanced processes for making said composite hollow fibermembranes than the prior art.

The following examples illustrate the present invention in details andare not intended to limit the same.

EXAMPLE 1 Effect of Multiple Coatings on Membrane Performance

All of the chemicals used were purchased from Aldrich Chemicals Inc.,Milwaukee, Wis. 53201.

A membrane casting solution (referred to herein as Dope I) was preparedby dissolving 13 parts by weight of poly(vinylidene fluoride) (PVDF), 5parts by weight of polyvinylpyrrolidone (PVP), 5 parts by weight ofaluminum chloride hexahydrate (AlCl₃.6H₂O), and 2 parts by weight ofpoly(vinyl butyral-co-vinyl alcohol-vinyl acetate) in 75 parts by weightof 1-methyl-2-pyrrolidinone (NMP) as a solvent.

A composite hollow fiber membrane was prepared by coating a tubularbraid with the above dope using a spinneret shown in FIG. 1 and aprocess depicted in FIG. 3. Ultrasonic sonication was applied to thecoagulation bath to speed up phase inversion process. The braid wascoated at a speed of 60 ft/min, coagulated and leached at 50-55° C. inwater to give a composite hollow fiber membrane, which was collected bya take-up wheel immersed in water at ambient temperature.

The membrane obtained above was characterized by measuring its diameter,burst pressure, water permeability, and by filtration of wine, milk,soymilk, orange juice, lemon juice, surface water, and sewage water. Allfiltration tests were conducted at ambient temperature, 10 psitransmembrane pressure, a liquid was allowed to flow from outside thehollow fiber membrane into its lumen to give a permeate. The orangejuice, lemon juice, milk and soymilk were purchased from a localsupermarket. The surface water was obtained from Canobie Lake, Salem,N.H. Sewage water was obtained from a local septic in Salem, N.H. Theresults obtained are summarized in Table 1. TABLE 1 COMPOSITION OFMEMBRANE CASTING SOLUTION (DOPE I) Poly(vinylidene fluoride) (PVDF) 13%Polyvinylpyrrolidone (PVP) 5% Aluminum chloride hexahydrate 5%(AlCl₃.6H₂O) Poly(vinyl butyral-co-vinyl alcohol-vinyl 2% acetate)1-Methyl-2-pyrrolidinone (NMP) 75% COATING CONDITION Dope Pressure 80psi 1^(st) coating Dope I 2^(ed) coating Dope I Coagulation bath Water,50-55° C. Primary leaching bath Water, 50-55° C. Secondary leaching bathWater, ambient temperature Coating speed 60 ft/min MEMBRANECHARACTERISTICS Braid outside diameter 63 mil Membrane outside diameter78 mil Burst pressure 40 psi Pure water permeability 50 gfd/psi CanobieLake water permeability 32 gfd/psi, permeate clear and potable Sewagewater permeability 11 gfd/psi, permeate clear and dischargeable Italianwhite wine: BELLA SERA 26.3 gfd/psi, permeate sparkling white wineFrench red wine: B & G MERLOT 7.9 gfd/psi, permeate sparkling red wineLemon juice permeability 1.1 gfd/psi, permeate clear Orange juicepermeability 0.88 gfd/psi, permeate clear, bright yellow Soymilkpermeability 0.79 gfd/psi, permeate clear, light yellow Milkpermeability 0.77 gfd/psi, permeate clear POST TREATMENT WITH 10,000 PPMNaOCl AT AMBIENT TEMPERATURE FOR 48 HOURS Water permeability 141 gfd/psiCanobie lake water permeability 38 gfd/psi, permeate clear and potableSewage water permeability 19 gfd/psi, permeate clear and dischargeable

EXAMPLE 2 Effect of an Adhesive Coating Layer on Membrane Performance

In example 2, a braid was first coated with a proprietary adhesive, thencoated with Dope I obtained from Example 1 using a spinneret displayedin FIG. 1 and a process depicted in FIG. 3 to give a composite hollowfiber membrane. The membrane was heated in an oven at 80° C. for 8 hoursbefore use. The condition used to make the membrane and membranecharacteristics are given in Table 2. TABLE 2 COMPOSITION OF MEMBRANECASTING SOLUTION (DOPE I) Poly(vinylidene fluoride) (PVDF) 13%Polyvinylpyrrolidone (PVP) 5% Aluminum chloride hexahydrate 5%(AlCl₃.6H₂O) Poly(vinyl butyral-co-vinyl alcohol-vinyl 2% acetate)1-Methyl-2-pyrrolidinone (NMP) 75% COATING CONDITION Dope Pressure 100psi 1^(st) coating Adhesive 2^(ed) coating Dope I Coagulation bathWater, 50-55° C. Primary leaching bath Water, 50-55° C. Secondaryleaching bath Water, ambient temperature Coating speed 60 ft/minMEMBRANE CHARACTERISTICS Braid outside diameter 63 mil Membrane outsidediameter 78 mil Burst pressure 82 psi Water permeability 17 gfd/psi Milkpermeability 1.2 gfd/psi POST TREATMENT WITH 10,000 PPM NaOCl AT AMBIENTTEMPERATURE FOR 48 HOURS Water permeability 42 gfd/psi Canobie lakewater permeability 18 gfd/psi, permeate clear and potable Sewage waterpermeability 8 gfd/psi, permeate clear and dischargeable

EXAMPLE 3 Effect of Adhesive Reinforcement on Membrane Performance

The composite hollow fiber membrane obtained from Example 1 was firstfilled with a proprietary adhesive formulated for strengthening thebinding between the support and the membrane, then drained to remove theexcess amount of adhesive from the membrane. The membrane was heated inan oven at 80° C. for 8 hours before use. The membrane obtained has thefollowing characteristics as shown in Table 3. TABLE 3 MEMBRANECHARACTERISTICS Braid outside diameter 63 mil Membrane outside diameter78 mil Burst pressure >100 psi Water permeability 20 gfd/psi Milkpermeability 1.2 gfd/psi, permeate clear POST TREATMENT WITH 10,000 PPMNaOCl AT AMBIENT TEMPERATURE FOR 48 HOURS Water permeability 55 gfd/psiCanobie Lake water permeability 22 gfd/psi, permeate clear and potableSewage water permeability 12 gfd/psi, permeate clear and dischargeable

EXAMPLE 4 Effect of Multiple Coating Layers on Membrane Performance

A membrane casting solution (Dope II) was prepared by dissolving 14parts by weight of PVDF-HFP, 5 parts by weight of PVP, 5 parts by weightof aluminum chloride hexahydrate, and 2 parts by weight ofPoly(acrylonitrile-co-vinylidene chloride-co-methylmethacrylate) in 74parts by weight of NMP as a solvent. A composite hollow fiber membranewas prepared by first coating a tubular braid with Dope II, followed bycoating with Dope II again using a spinneret shown in FIG. 1 and aprocess depicted in FIG. 3. The ultrasonic sonication was applied to thecoagulation bath, primary and secondary leaching baths to speed up phaseinversion process. The condition used to prepare a composite hollowfiber membrane is given in Table 4. The membrane obtained wascharacterized in the same way as in Example 1, and the results obtainedare summarized in Table 4. TABLE 4 COMPOSITION OF MEMBRANE CASTINGSOLUTION (Dope II) Poly(vinylidene fluoride-co- 14% hexafluropropylene)(PVDF-HFP) Polyvinylpyrrolidone (PVP) 5% Aluminum chloride hexahydrate5% (AlCl₃.6H₂O) Poly(acrylonitrile-co-vinylidene chloride- 2%co-methylmethacrylate) 1-Methyl-2-pyrrolidinone (NMP) 74% COATINGCONDITION Dope Pressure 100 psi 1^(st) coating Dope II 2^(ed) coatingDope II Coagulation bath Water, 50-55° C. Primary leaching bath Water,50-55° C. Secondary leaching bath Water, ambient temperature Coatingspeed 100 ft/min MEMBRANE CHARACTERISTICS Braid outside diameter 63 milMembrane outside diameter 78 mil Burst pressure 36 psi 200 kD PEOrejection 90.0% Water permeability 26 gfd/psi Lemon juice permeability0.96 gfd/psi, permeate clear Orange juice permeability 0.81 gfd/psi,permeate clear, bright yellow Soymilk permeability 0.69 gfd/psi,permeate clear, light yellow Milk permeability 0.72 gfd/psi, permeateclear POST TREATMENT WITH 10,000 PPM NaOCl AT AMBIENT TEMPERATURE FOR 48HOURS Water permeability 51 gfd/psi Canobie Lake water permeability 27gfd/psi, permeate clean and potable Sewage water permeability 10gfd/psi, permeate clean and dischargeable

Although the composite hollow fiber membranes are used to illustrate thepresent invention, the formulations, methods and processes discovered inthe present invention are applicable to flat sheet composite membranes,large diameter tubular composite membranes and any other compositemembrane having a different geometry.

1. A method for strengthening a multi-layer defect free compositemembrane, wherein said method comprising impregnation of a support witha binding agent while leaving the membrane surface free of said bindingagent, followed by curing said binding agent either at ambient or atelevated temperature with or without the presence of ultraviolet lightdepending on desired membrane performance.
 2. The method according toclaim 1, wherein said binding agent is selected from the groupconsisting of epoxy, polyurethane, silicone, any other adhesive and anyother organic or inorganic material which has excellent compatibilitybetween said support and said membrane to chemically or physically bondthem together to prevent said membrane from delaminating from saidsupport.
 3. The method according to claim 1, wherein said methodincludes an optional post treatment of said membrane with ozone or asolution containing 100-150,000 ppm free chlorine at ambient or elevatedtemperature to increase membrane liquid or gas permeability by 2 to 10folds compared to a virgin membrane never exposed to any post treatment.4. The membrane products produced according to the method of claim 1.